1. Field of Application
This invention relates to encoded symbology; and, more particularly, to assessing or verifying, the quality of such encoded symbology after it has been applied to a carrier.
2. Description of the Prior Art
The preferred encoded identification [symbol/symbology] marking for use with this invention is the two-dimensional (2-D) matrix symbol particularly the one referred to as “DATA MATRIX”.
One and two dimensional part marks have achieved wide spread acceptance throughout a wide variety of industries. 2-D matrix symbol/symbology was developed to overcome many of the deficiencies inherent in the first-generation (linear bar codes) and second-generation (stacked bar codes) symbol formats. One of the primary changes was the use of squares or rectangles as a carrier of data in lieu of the strips of variable widths used in linear and stacked bar codes. The use of data elements of known size and shape makes 2-D matrix codes more versatile. In the matrix code format, black data elements (cells) usually represent a binary “1” and white data elements (cells) usually represent a binary “0”. When these binary values are used together in specific sequences, they represent alphanumeric characters. Matrix symbols can not only be produced in both square and rectangular format but they can also be scaled in size to fit into an available marking area.
Matrix codes, designed to be applied to any of a variety of articles, parts and products, are described, for example, in U.S. Pat. No. 4,939,354 (issued Jul. 3, 1990 to D. G. Priddy, et al.). A matrix code can store from one to 2335 alphanumeric characters in any language. An encoding scheme for use with such a symbol has a high degree of redundancy that permits most marking defects to be overcome. 16-bit cyclic redundancy check and data reconstruction capabilities are included in one version; and Reed-Solomon error correction is included in another. Up to 16 symbols can be concatenated. Error correction and checking (ECC) code 200 is possible.
The term “DATA MATRIX” has been certified by AIM-USA and AIM-International as a fully public-domain symbology. AIM stands for Automatic Identification Manufacturers International, Inc. “DATA MATRIX” is a unique machine readable symbol capable of storing a large amount of information within a small physical size. The data matrix symbol allows for two-dimensional encoding and decoding. Users are not constrained by the limitations of a printed symbol. Data matrix symbols are capable of carrying 25 to 100 times more information than the typical barcode. This range is directly related to the image quality the printer is capable of producing. “DATA MATRIX” codes have the following characteristics: both height and width are used to encode data; they work with contrast as low as 20%; they are readable through 360. degrees. of rotation; they are designed to survive harsh industrial environments; such codes are often printed on a substrate such as paper but they can be marked directly on the surface of a part, without using a paper label or substrate.
Some, systems and devices for reading such one and two dimensional symbols begin by determining the orientation of the markings before trying to read the symbol. Usually this is done by locating an outer reference bar(s) or a central symbol. Once the orientation of the marking is determined, the marking is read. and several error correction schemes are available to ensure damage recovery. It has, however, become important in many applications to verify the quality of the encoded symbology; including encoded symbology of the “DATA MATRIX” type.
A method for verifying “DATA MATRIX” print quality is shown and described in U.S. Pat. No. 6,244,764 patented on Jun. 12, 2001 to Ming Lei et al for “Method for Data Matrix Print Quality Verification”. The method is described for verifying 2-D encoded symbology print quality on all types of direct part and label marking applications. The described method measures symbol contrast, print growth, axial non-uniformity, unused error correction, and overall grade. In addition to these parameters, the cell placement accuracy, cell size uniformity, and overall symbol quality are also measured. The method may also provide other relevant information about the data matrix, such as polarity, symbol size, error correction level, image style, and encoded data string. However, even the symbol quality indications resulting from using the method of this patent does not satisfy some industrial, commercial and/or government symbol quality requirements.